The present invention relates to air data sensing systems that provide accurate compensation for measurement errors caused by sideslip of an air vehicle utilizing multi-function air data probes that are not pneumatically coupled, but which have processors for interchanging electrical signals between the multi-function probes. Compensation of pressure signals such as those measured at a number of discrete locations on a multi-function probe during sideslip of the air vehicle is necessary to provide true angle of attack and other aircraft parameters including altitude and airspeed. The requirement for accuracy in altitude indications is particularly important in Reduced Vertical Separation Minimum (RSVM) areas of the air traffic control system.
Conventional air data sensing systems have probes on opposite sides of an aircraft pneumatically connected so that the pressure signals are averaged between the right and left sides of the aircraft to provide a static pressure signal that is xe2x80x9cnearly truexe2x80x9d. In most conventional systems, with pneumatically connected ports, corrections are made for Mach number and aircraft angle of attack, but it is rare that it is necessary to introduce further corrections for the effects of sideslip on static pressure.
However, SmartProbe(trademark), multi-function probes which have processors as part of the instrument package right at the probe, are connected only electrically (digitally) in order to eliminate the need for pneumatic tubing passing between the opposite sides of the aircraft or between multi-function probes on the same side of the aircraft. The term xe2x80x9cprobesxe2x80x9d is used to designate such multi-function probes. This means that each probe is pneumatically independent even if it is electrically xe2x80x9ctalkingxe2x80x9d to another probe. In the RVSM airspace, there needs to be highly accurate dual redundant systems in place for static pressure estimations. While information can easily be exchanged between the processors of different probes, the need for determining sideslip effects remains. Computational fluid dynamic analysis and flight testing has shown that position errors can be up to 600 feet per degree of sideslip under typical RVSM flight conditions, for example at 41,000 feet and Mach equal to 0.8. It is thus apparent that the sideslip effect must be corrected for to obtain the necessary accuracy for certification.
The present invention relates to multi-function air data sensing systems which provide highly accurate redundant measurement capabilities of various air data parameters, such as angle of attack, pressure altitude, and Mach number. Aerodynamic sideslip is a measure of the magnitude of a cross component of airspeed to the forward component of airspeed. Inertial navigation systems will measure yaw angle that is very nearly that of the aerodynamic sideslip at high forward speeds, but such inertial measurements take into account the orientation of the aircraft, and not the wind speed, although inertial inputs can be utilized for compensation of errors in the pressure measurements from air data sensing probes. Additionally, however, compensation information exchanged between multi-function probes, such as differential and local angle of attack between the two sides of an aircraft, can provide an indication of sideslip, utilizing the system disclosed herein.
Redundancy can be obtained in one form of the invention with symmetric probes by utilizing a third forward facing air data sensing probe having its central axis on the center line of the aircraft and, at the forward end of the aircraft. The probe used for the center line probe has ports that will measure pitot pressure and local angle of sideslip. Sideslip is measured by ports on opposite sides of a vertical plane along a central aircraft axis. The angle of sideslip ports are also centered on a plane that is oriented horizontally, or perpendicular to the center plane when the aircraft is oriented to be at a level position (0xc2x0 angle of attack). The compensation values avoid the need for inertial yaw information, and yet provide redundancy, eliminating a fourth multi-function probe, as is now used to have a totally redundant system. A four probe system has two pairs of symmetrically located probes, with one probe of each pair located on the opposite sides of the aircraft. This would provide two independent measures of local, sideslip compensated pressures. The use of the local sideslip reading from the centerline probe now can be used as input to compensate the local static pressure for aerodynamic sideslip. The local sideslip value from the centerline probe, coupled with two additional measures of local static pressure now give the same two systems as the pneumatically averaged systems. In each case, there are two readings of sideslip compensated local static pressure.
There is a strong correlation between the local sideslip angle that is measured by the center line probe, and the actual aircraft sideslip when the angle of attack is held constant. Using the non-dimensional pressure difference between the zero sideslip case and a non-zero sideslip case (Cp@xcex2=0-Cp@xcex2=0) as a dependent variable, there is a direct linear dependence on the local sideslip angle measured at the centerline probe. It is shown to be true under all aircraft angles of attack. Thus, a direct indication of the effects of sideslip on pressure altitude is achieved without the need for iteration or complex algorithms that would solve for accurate pressure altitudes based on pressure inputs.
When probes are mounted either asymmetrically, or on the same side of the aircraft, simple electrical averaging of the pressures is not likely to provide sufficient accuracy, but it has been found that utilizing information of other locations can be adequate to correct for sideslip errors in the measurements made directly from the probes in use when more complex algorithms are used.
In many installations, the differential angles of attack measured between probes symmetrically located on opposite sides of the aircraft provides a direct indication of sideslip. In some combinations of locations, and some aircraft angles of attack, pressure sensitivity to sideslip can approach zero, but as long as there is sensitivity to sideslip in the measurement of angle of attack between two probes, this indication can be used.
Compensation for sideslip can also be obtained by using a flush mounted static pressure sensing plate with a static port to add an additional pressure with which to compensate for angle of sideslip. Flush plates, symmetrical to one of the multi-function probes is a preferred solution. In other words, with a multi-function probe on one side of the aircraft, a flush static sensing plate would be mounted symmetrically on the opposite side of the aircraft.